Make sure the default Apache virtual host points at your WordPress MU site so it will handle unknown domains correctly. (Need info on cpanel, etc. How do you get them to respond to any domain?)

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I guess what they mean by default vhost, is the vhost that gets called if none other fits!? does that make more sense?
I.e. a domain points to this server and then you delete it from the server but the DNS still points here, so where do you land when accessing this domain?

I am playing with wpmu, thats what wordpress.com is using. with this plugin, people can sign up for i.e till.mydomain.com and redirect their own domains i.e. till.de to till.mydomain.com and my wpmu isntallation will catch this so you use your own domain i.e. till.de but use my services at till.mydomain.com.

does that make sense to you? I tried the long explanation, but your one sentence

Do you want to redirect subdomains of a domain or different domains to your wordpress installation?